We are working to make our Terms of Service agreement easier to read in order to provide you with better services while protecting your personal data. We hope you can spare a few minutes to read the recently updated User Terms of Service.
User Terms of ServicePrivacy Policy

is this bug on burnishing artifact?

You cannot keep burnishing 1 stat & ignore others.
First you need to get ALL 5 slots to aura +1.
When they all are Aura +1, then you can burnish then to Aura +2.
When all 5 slots are at Aura +2, then you get to burnish Aura +3.
...And so on.